1. Field of the Invention
The present invention relates to measurement of the optical characteristics of an object.
2. Description of the Related Art
The color and gloss of an object are represented by diffuse light (color) colored in a coating film on the surface of the object or inside the object, and specular reflection light (color) of illumination light reflected by the coating film or the object surface. As for the surface of a printed material, diffuse light exhibits relatively the same isotropy when viewed from any direction. To the contrary, reflected light changes depending on the incident direction (illumination direction) of illumination light with respect to the surface of a printed material, and the exit direction (observation direction) of the reflected light.
To obtain the characteristic of the color (and gloss) of an object in an arbitrary illumination direction and observation direction, the object is irradiated with illumination light from various directions to measure light (color) radiating from the surface of the object omnidirectionally to the periphery of the object. The measurement result is called a bidirectional reflectance distribution function (BRDF). As the radiation distribution characteristic of reflected light from an object to the periphery, the BRDF is used in various applications such as computer graphics (CG).
There is known the first technique of acquiring the BRDF of an object by radiating light at an arbitrary portion on a hemispherical surface around the object and receiving it at an arbitrary portion on the hemispherical surface. According to the first technique, the BRDF of an object can be acquired. However, the BRDF obtained by one measurement is merely the measurement result of one point on the reflecting surface of the object. A very long measurement time is required to measure an object whose BRDF changes depending on the location.
There is also known the second technique of acquiring BRDFs from a plurality of two-dimensional images obtained by scanning a measurement target surface with a light source of line shape and capturing reflected light beams at respective scanning positions with a camera. According to the second technique, the BRDFs of respective points on a measurement target surface can be measured by one scanning and a plurality of image capturing operations.
When only the color of a measurement target surface, that is, the color of diffuse light (to be referred to as “diffuse color” hereinafter) is acquired, measurement ends by only one capturing if a two-dimensional image sensor (for example, a camera apparatus) is used. Even when the color is acquired by a line sensor (for example, a scanner apparatus), measurement ends by a line sensor scanning length equal to the length of the measurement target surface. However, even if the second technique is employed, the acquisition of the BRDF requires a much longer measurement time, compared to the acquisition of only the color (diffuse color), because the scanning length of the light source of line shape becomes double or larger than the length of the measurement target surface.